Presentation on theme: "Plus, the density formula and Avogadro’s Law"— Presentation transcript:
1 Plus, the density formula and Avogadro’s Law The Ideal Gas LawPlus, the density formula and Avogadro’s Law
2 Review Gases are made of particles that move rapidly and randomly. Temperature is a measure of how rapidly the molecules in a gas are moving on average.Collisions between gas molecules surface of an object (or the walls of a container) give rise to gas pressure.Standard temperature and pressure273 K (0ºC)kPa (1 atm)
3 Ideal GasesAn ideal gas is one that perfectly obeys the predictions made by the KMT:Its molecules have zero diameter.Its molecules have zero intermolecular forces.Collisions are always elastic.Gases aren’t perfectly ideal.Some gases approach “idealness” under certain conditions:high temperaturelow pressure
4 Ideal vs. Real GasesThe boiling point of nitrogen (N2) is Kelvins ( ºC).At room temperature (298 K) and standard pressure (1 atm), nitrogen behaves very much like an ideal gas.Room temperature is far above nitrogen’s boilling point.At 78 K and 30 atm, nitrogen’s behavior isn’t so ideal.Its molecules are just barely moving fast enough to remain in the gas state.Intermolecular forces affect the behavior of N2 at such a low temperature.High pressures squeeze the molecules close together, increasing the effects of their intermolecular forces.
6 Ideal vs. Real GasesIn a gas that approximates ideal behavior, molecules are fast-moving and far apart.At very low temperatures, gas molecules become “sluggish” and attractive forces alter their behavior.At very high pressures gas molecules are squeezed close together and molecular interactions become far more common (and important).
7 The Ideal Gas Law PV = nRT P = pressure (atm)V = volume (L)n = molesR = Universal Gas ConstantR = L*atm/mol*KT = temperature (K)Applies to gases that exhibit ideal behavior.For non-ideal gases (gases at very low temperature or extremely high pressures) PV ≠ nRT.
8 The Ideal Gas Law In order to use R = 0.0821 L*atm/mol*K, Pressure must be expressed in atm.Volume must be expressed in L.Temperature must be expressed in K.You should get used to converting between different units of pressure, volume, and temperature.There are other values of R for use with other units, but instead of learning many different values for R, you should learn how to convert.
9 The Ideal Gas LawWhat is the volume of 3.00 moles of helium at a temperature of 400. K and a pressure of 1.50 atm?PV = nRT(1.50 atm) V = (3.00 mol)( L*atm/mol*K) (400. K)(1.50 atm) V = 98.5 L*atmV = 65.7 L
10 The Ideal Gas LawWhat is the pressure exerted by moles of nitrogen gas in a 45.0 L container if the temperature is 350.ºC?First we must convert ºC to K?350.ºC = 623 KPV = nRTP(45.0 L) = (0.400 mol)( L*atm/mol*K)(623 K)P(45.0 L) = 20.5 L*atmP = atm
11 The Ideal Gas LawAt what temperature (in ºC) will 2.00 grams of argon gas exert a pressure of 12.5 atm in a 160.-mL container?First we must convert from grams of Ar to moles:2.00 g Ar x = mol ArNext, convert from mL to L:160. mL x = Lg Ar1 mol Ar1000 mL1 L
12 The Ideal Gas LawAt what temperature (in ºC) will mol of argon gas exert a pressure of 12.5 atm in a L container?PV = nRT(12.5 atm)(0.160 L) = ( mol)( L*atm/mol*K) T2.00 atm*L = ( L*atm/K) TT = 486 KT = 213º C
13 Gas Density Density = mass / volume The volume of an ideal gas is: V = nRT / PPutting the two equations together:D = mP / nRTThe molar mass of a gas is the number of grams (m) per mole (n), so:Molar mass (M) = m/nFor an ideal gas, D = PM / RT
14 Gas DensityWhat is the density of chlorine gas, Cl2, at atm and 300. K?The molar mass of Cl2 is:2 x Cl = 2 x g/mol = g/molD = PM / RTD = (0.950 atm)( g/mol) / ( L*atm/mol*K)(300. K)D = 2.73 g/L1 Liter of Cl2 under these conditions would weigh 2.73 grams.
15 Gas Density What is the density of CO2 gas at STP? D = PM / RT The molar mass of CO2 is:1 x C = 1 x g/mol = g/mol2 x O = 2 x g/mol = g/molTotal = g/molStandard temperature = 273 KStandard pressure = 1.00 atmD = PM / RTD = (1.00 atm)( g/mol) / ( L*atm/mol*K)(273 K)D = 1.96 g/L
16 Avogadro’s LawIn addition to having a very large number named after him, Amedeo Avogadro made a very important deduction about gases.Equal volumes of ideal gases at equal temperatures and pressures contain equal numbers of molecules.Doesn’t matter what the identity of the gas is.In other words, if you have two different ideal gases under identical conditions, the molar volume of the gases is the same.At STP, 1 mole of any gas = 22.4 L
17 Avogadro’s LawWhat is the volume of 2.00 moles of O2 at STP? (Assume ideal behavior.)1 mol of any STP = 22.4 L2.00 mol O2 x = 44.8 L O21 mol O222.4 L O2
19 A ReminderGas law problems don’t always have to come with easy-to-use units.They often require unit conversions before they can be solved.You should always ask yourself “What does this problem want me to find, and what am I given?”Make a plan to solve the problem based on the available information.Not every problem will be the same.If you approach all gas law problems expecting them to be identical, you will be frustrated.Don’t get lazy or sloppy in your work!
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